The objectives of this grant are to investigate lipid metabolism during the cell cycle in E. coli and to investigate the significance of these changes. Two approaches have been employed to obtain information regarding the division cycle in E. coli K12 and B/r. The fractionation of an inactivated log population into size classes using sucrose gradients and subsequent analysis provide information during an undistrubed division cycle. During pulse-label experiments with both 32PO4 and with 14C-acetate, the relative abundance of newly synthesized phospholipids, fatty acyl chains, and phospholipid molecular species remained constant. The rate of synthesis of lipids was examined by continuously labelling with 3H-isoleucine and pulse-labelling with either 32PO4 ions or 14C-acetate for lipid. Lipids were extracted and washed and the rate of lipid synthesis examined per unit of protein. The rate of lipid synthesis increased near the time of appearance of cross-walls in both K-12 and B/r strains. Cells were uniformly labelled with both 3H-isoleucine and 32PO4 ions to determine the lipid to protein ratio. Although the scatter in this data is still quite significant, our results indicate that small cells and cells in the final stage of cell division have 50% to 60% more lipid than do cells in the intermediate stage of division. The fluidity of membranes from baby cells, mid-cycle cells, and dividing cells was examined by fluorescence polarization. No changes were observed using diphenylhexatriane as a approbe. However; further subdivision of the cell cycle may be necessary to eliminate this possibility.